Contemporary cellular mobile telephone systems conventionally include a "diversity" system of some nature in the base or fixed station receivers to reduce the effects of multipath fading wherein large rapid fluctuations of signal level occur. In such diversity systems the signals received by two or more antennas are normally made available to the receiving equipment and a process or scheme is provided whereby when the signal from one antenna is experiencing a large negative fluctuation or deep fade the signal from another antenna may be substituted. Such switching thus produces a signal level of higher average signal strength, and the quality of communications is inherently improved.
A typical such diversity system associated with receiving equipment is illustrated in FIG. 1 which is a simplified block diagram illustrating a single channel. In a typical such system each antenna conventionally may serve many channels through a multicoupler. Although such multicouplers are not illustrated in subsequent figures, the exemplary embodiments found therein may be assumed to include such multicouplers. As illustrated in FIG. 1, such systems are provided with two receivers per RF channel, and each receiver has an antenna input and audio output as well as a received signal strength indicator (RSSI) output. The latter output is an analog output which is indicative of or provides a measure of the received RF signal strength at the antenna input. A comparison of the RSSI receiver outputs is made to determine the larger of the two, and the generated high or low signal may be used to operate a switch such as SW1 so as to select the input for the audio processing element from the receiver with the highest RSSI signal. Such a diversity scheme is known as a "post-detection receiver selection by RSSI" and is in common use.
It is also known to use the RSSI or "received signal strength indicator" for other purposes such as determining when it is necessary to hand off or transfer the communications function to another cell. Such a determination may be made by converting the RSSI measure to a digital format for transmission to a central processor for comparison with similar signals from other cells so that the cell having the strongest signal from the mobile unit may be determined. The RSSI signals may be used for these and other purposes through the use of a sampling switch means (SW2), an analog-to-digital converter and the logic unit as illustrated in FIG. 1.
Diversity circuits where the instantaneously larger of the input signals is passed to the receiver, operate with sufficient speed to follow the rather large and rapid fluctuations of the received signal strength that are present due to multipath fading. Determining when to transfer or hand off the communications function to another cell, however, is not dependent on rapid fluctuations but is dependent on the mobile position and average signal strength. Accordingly, both instantaneous and average signal strengths must be determined.
As seen in FIG. 1 two receivers and two antennas are required for each area to be covered. However, in areas where the number of subscribers is quite high it is necessary to provide smaller cells and re-use frequencies in cells that are relatively close to each other in order to provide a sufficiently high number of channels to handle the communication traffic. In such areas it is conventional to subdivide cells into sectors and use directional antennas to reduce interfering signals from other cells. Each sector would require two antennas and two receivers for maintaining diversity as noted above. Where a cell, for example, is divided into three 120.degree. sectors, six receiving antennas would be required. When the cell is further divided, maintaining diversity would lead to a prohibitively large number of antennas and overcrowding of support towers and the like. Clearly, under such circumstances it would be desirable to provide the benefits of diversity but with fewer antennas being required in each sector of a cell.
We have discovered that effective diversity with only one receiver antenna in each sector of a sectored cell may be obtained through the use of "adaptive" diversity. Such diversity dynamically adapts to changing signal conditions wherein an analog comparator accepts RSSI signals from two receivers and the receivers operate to receive the signals from three sectors of a sectored cell; one receiver being connected to the center sector antenna and the other receiver being switchably connected between left and right adjacent sector antennas. Effective diversity is maintained (for a large portion of the time) between the center sector and one of the adjacent sectors. For very short periods of time the audio is taken from the center sector only while the second receiver measures RSSI in the other adjacent sector. By keeping track of average signal levels in both adjacent sectors we can adaptively determine which adjacent sector has the largest average signal and use it during the large time period when diversity is in effect.
Such adaptive diversity method and apparatus not only maintains diversity where large rapid fluctuations of signal level occur but also dynamically adapts to changing conditions due to movement of the mobile unit. Thus, the shortcomings of the prior art are overcome.